Insulin is one of peptide hormones that are secreted from pancreatic β-cells in the pancreas, and acts to decrease an elevated blood glucose level and keep it at a normal level. Main physiological effects of insulin may include promotion of uptake of sugars, amino acids and the like and protein synthesis in the muscle tissue; and promotion of sugar uptake and utilization thereof, promotion of lipid synthesis, suppression of lipid decomposition or burning, and promotion of protein synthesis in the adipose tissue; and the like.
Secretion of insulin is mainly promoted by glucose. When sugar is uptaken into a body by meals or the like and a blood sugar level (blood glucose level) increases, insulin is secreted so as to decrease the elevated blood sugar level, as a result, the blood insulin level increases. Therefore, the secretion of insulin is very important for keeping a blood sugar level at a constant value to prevent diabetes mellitus.
On the other hand, it is known that continuous secretion of insulin under a state of hyperglycemia leads to decrease of insulin sensitivity (insulin resistance) in the skeletal muscles, liver and adipose tissue that are target organs for insulin. When insulin resistance occurs, insulin is secreted more from the pancreas so as to compensate for insufficiency in an effect of decreasing blood sugar. When such excess secretion of insulin is repeated, the pancreas gets exhausted, and finally the ability of secreting insulin from pancreatic β-cells is decreased, while keeping higher insulin resistance in the target organs. The above functional deterioration of regulatory mechanism of insulin in the body may leads to development of lifestyle diseases such as diabetes mellitus, and further obesity, Type II diabetes mellitus (hypertension) and the like (see Non-patent Literature 1).
Until today, it has been considered that an amount of insulin secretion in the blood varies depending on blood sugar levels, i.e., uptake amounts of carbohydrates. However, in recent years, it has been newly reported that lipid uptake also correlates the elevation of blood insulin level as well as carbohydrate uptake (see Patent Literature 1). According to Patent Literature 1, it was confirmed that when carbohydrates and lipids are uptaken together, a secretion amount of insulin is excessively higher than that of when carbohydrates are uptaken solely. Further, it was also confirmed that the excess secretion of insulin due to such simultaneous ingestion of carbohydrates and lipids is a factor which is highly correlated to obesity.
In recent years, eating habits of Japanese has been westernized, and meals have been changed from conventional meals including mainly carbohydrates to meals rich in lipids. As a result of such change in eating habits, increase in lifestyle diseases and metabolic syndrome has become a problem. Therefore, it is important to prevent and improve adverse effects of high fat diets on health.
In order to prevent diabetes mellitus, various approaches for suppressing elevation of blood sugar levels have been made. For example, Patent Literature 2 suggested an agent for improving blood sugar levels using polyglutamic acids so as to suppress elevation of blood sugar levels. Polyglutamic acids are widely used as moisturizing agents, absorbing agents and the like due to their high water retaining ability, and gain attentions as biodegradable polymers. Further, it was reported that polyglutamic acids have an effect of promoting absorption of calcium from the small intestine, an effect of suppressing elevation of blood pressure, and an effect of promoting saliva secretion (for example, see Patent Literatures 3 to 5).
However, under the current circumstance, almost no active effort is undertaken for suppressing elevation of blood insulin concentration.